Garments made from aramid fibers or aramid filaments are known in the art of personal protection equipment for conferring excellent protection against radiant heat, fire and electric arcs, as well as for their excellent mechanical properties.
For this reason, garments comprising the aforementioned aramid materials are extensively used in many protective garments, such as industrial protective garments, fire fighter gear, police or military uniforms.
The use of aramid fibers or filament in such garments or uniforms implies that such fibers or filaments must sometimes be extensively colored, not only for aesthetic purposes but also often out of necessity to be clearly visible. Other examples include the coloring for military-purpose garments such as for example camouflage and specific IR reflectance needs.
For coloring aramid materials, there exist multiple dyeing procedures, most of which use synthetic dyes. Generally, such dyeing processes involve heating the aramid material in an aqueous solution of cationic dye, carrier substances and inorganic salts, followed by a one or more rinsing steps. Other coloring routes include the incorporation of pigment in the polymer solution before the spinning process, known as spun-in pigments.
However, such synthetic dyes and pigments are produced on an industrial scale, and may sometimes comprise residual amounts of undesirable compounds used in the synthesis of such dyes. Such residual compounds are generally present in low amounts and include, for example, different isomers of dichlorobenzene and derivatives thereof.
It is desirable to reduce the levels of such compounds in finished aramid material without adversely affecting or washing away finishing compounds that were purposefully applied to the aramid material, such as for example fluorinated finishing compounds.
It is furthermore desirable to reduce the levels of such compounds in aramid material without negatively affecting the mechanical properties of the aramid material, or even more desirable to reduce the concentration of such levels in aramid material by simultaneously improving the to mechanical properties of the aramid material.
Furthermore, there is a constant desire to increase the heat, flame and electric arc resistance properties of the aramid material in order to manufacture improved personal protection equipment that protects more effectively while using the same amount of aramid material. In addition, it is also desirable to keep, or even enhance the dimensional stability of the aramid material as well as other wanted features such as for example increasing the crystallinity of the fiber which influences directly on mechanical and thermal stability.
The crystallinity of the aramid fiber may be increased above 15%, when measured by Raman spectroscopy as described in WO2011011395, by applying heat and tension to the fiber, e.g. by stretching the fiber at a temperature above its glass transition point, or by chemically treating the fiber in a coloring, dyeing or mock dyeing process with dye carriers such as benzyl alcohol and benzophenone. Examples of such methods may be found in U.S. Pat. Nos. 4,668,234, 4,755,335, 4,883,496 and 5,096,459. Such mock dyeing methods typically employ carrier components and salt components in considerable concentrations, which renders them economically undesirable. There exists therefore a need to reduce the use of such costly components, which also need to be disposed of in an environmentally sustainable method, in order to provide for a more cost-effective process.
Other known methods that increase crystallinity in meta-aramids comprise the step of heating the fiber, preferably in the form of a tow, in steam at temperatures between 140° C. and 165° C. While aramid fiber and especially meta-aramid fiber has good heat resistance properties, the dyes commonly used to dye aramid fibers have not. Thus, it is not so far not practically feasible to increase crystallinity by heat in a dyed aramid fiber without thermally degrading the dye.